So-called “Re-Breather circuits” (otherwise known as “circle systems”) are well known anaesthetic circuits for delivering anaesthetic to patients.
Anaesthetic circuits deliver a mixture of oxygen (O2) and anaesthetic to a patient. A Re-Breather circuit recycles unused O2 and anaesthetic and removes unwanted carbon dioxide exhaled by the patient.
Re-Breather circuits are widely used, and their operation well understood. A typical Re-Breather circuit is supplied by Fresh Gas (a mixture of O2 and anaesthetic) from a vaporiser. The Re-Breather circuit includes two hoses and two one way valves which connect to the patient and enable the patient to first inhale O2 and anaesthetic from the inhalation side of the circuit and to then exhale CO2, and unused O2 and anaesthetic back into the exhalation side of the circuit—thus ensuring a one way flow of gases through the hoses from the inhalation side to the exhalation side of the circuit.
The Re-Breather circuit additionally contains a soda lime canister filled with soda lime, a breather bag and a pressure relief valve. The exhaled gases pass through the soda lime where the CO2 is absorbed before they are recycled back to the inhalation side of the circuit to be Re-Breathed by the patient. The breathing bag acts as a capacitor in the circuit to absorb or dampen the pressure fluctuations that would otherwise occur when the patient inhales and exhales.
The pressure relief valve has two functions. Firstly it ensures that the Re-Breather circuit and the patient attached to this circuit is not pressurised to a level that is dangerous to the patient. Secondly it maintains the mass of gases contained in the Re-Breather circuit approximately constant. In this respect the circuit is constantly fed with a mixture of new O2 and anaesthetic (Fresh Gas) to provide replacement anaesthetic for that absorbed by the patient and replacement O2 for that converted to CO2 by the patient and subsequently absorbed by the soda lime. In order to maintain the mass of gases in the Re-Breather circuit approximately constant the pressure relief valve must release a mass equal to the mass of Fresh Gas delivered to the Re-Breather circuit minus the mass of CO2 absorbed by the Soda Lime and the mass of anaesthetic gas absorbed by the patient.
There are a number of issues with Re-Breather circuits and they are not necessarily ideal for all types of patients.
One problem associated with Re-Breather circuits is a poor dynamic response to changes in patient demand for increased anaesthetic concentration. The need for rapid increase in the anaesthetic concentration of the patient typically occurs when the patient starts to wake up during an operation. Rapid increase in the anaesthetic concentration delivered to the patient is required if the patient is to be put back to sleep. Poor dynamic response (depending, in part, on patient type) can mean that the patient may not be put back to sleep in appropriate time, or receive the right amount of anaesthetic to keep them at the correct anaesthetic depth.
Re-Breather circuits also have a certain amount of resistance to breathing, which may not be suitable for all types of patients.
Although currently available Re-Breather circuits may perform satisfactorily to maintain anaesthetic depth of adult sized human patients e.g. 40 kg and over, there are issues with their use by smaller patients, particularly under 15 kgs. Re-Breather circuits are used in veterinary medicine as well as human medicine. Because of the difficulties with smaller patients, such, as small animals, and the impracticality of designing Re-Breather circuits particularly for smaller patients, typically small patients such as small animals are not anaesthetised using Re-Breather circuits. Instead, such patients are anaesthetised using “open circuits” (or “None Re-Breather circuits”). Open circuits are high-flow systems. Large amounts of anaesthetic, in the order of a liter per minute in some cases, are used. This is very expensive. Also, a significant amount of anaesthetic leaks into the surrounding environment from the open system, causing a potential health hazard. The active provision and removal of anaesthetic/gas mixture is therefore very wasteful and expensive. With open systems, it is also very difficult to ensure with any precision that a patient is anaesthetised. It is difficult to tell with any accuracy the concentration of anaesthetic gas mixture at the patient respiratory orifices. There will be, for example, entrainment of room air because of the open system, which can dilute the anaesthetic gas mixture. It can therefore be very difficult to maintain anaesthetic depth with open circuits.